Ventricular tachycardia is a disease of the heart in which the heart's normal rhythmic contraction is altered, thus affecting heart function. The condition is often described as a heart beat which is too fast, although the disease is far more complex. Ventricular tachycardia occurs most often in patients following a myocardial infarction. A myocardial infarction, commonly referred to as a heart attack, is a loss of blood flow to a region of the heart causing the myocardial (muscle) tissue in that region to die and be replaced by an area of scar tissue known as a myocardial infarct. In most cases, this occurs in the left ventricle.
Ventricular tachycardia ("VT") may be initiated and sustained by a re-entrant mechanism, termed a "circus" movement. The mechanism of re-entry, as it is currently understood, is discussed in M. Josephson and H. Wellens, Tachycardias: Mechanisms, Diagnosis, Treatment, Chap. 14 (1984) (Lea & Febiger). Most cases of sudden cardiac death that have occurred during cardiac monitoring have begun as VT that degenerated into ventricular fibrillation.
While VT can be halted after it begins by pacing or cardioversion, it is preferable to prevent the arrhythmia from arising. Drug therapy has been used, but is successful in only 30 to 50 percent of patients and has undesirable side effects. Endocardial resection, a surgical procedure involving removing the tissue in the ventricle thought to be the source of the VT, has been reported to eradicate VT in up to 90 percent of patients, but it suffers from a 5 to 10 percent incidence of perioperative mortality. For a discussion of surgical procedures, see T. Ferguson and J. Cox, Surgical Therapy for Cardiac Arrhythmias, in Nonpharmacological Therapy of Tachyarrhythmias (G. Breithardt et al. eds. 1987).
As an alternative to surgery, the technique most often attempted is ablation. Typically, programmed premature pacing is performed from a catheter electrode in the right or left ventricular cavity. During programmed premature pacing, a stimulus, usually of twice diastolic threshold, is repeatedly given prematurely until either VT is induced or the tissue is too refractory to be excited. The ECG is examined during induced VT and compared to the ECG showing spontaneous bouts of VT. If the ECG is similar, it is assumed that the patient's clinical VT is being induced. A mapping catheter in the left ventricular cavity is used to record from numerous sites sequentially to determine the activation sequence along the left ventricular endocardium during the induced VT. The site from which activation appears to originate during the induced VT is identified and assumed to be a portion of the reentrant pathway. The techniques of pace mapping and entrainment may then be used in an attempt to confirm or refine the localization of the region rising to VT. The region is then ablated. Unfortunately, this technique is usually unsuccessful unless repeated many times. For example, it has been reported by Downar et al. that for a similar technique (the electrodes were located on an endocardial balloon instead of a catheter), anywhere from 10 to 42 shocks through different electrodes were required to prevent the reinduction of VT. It is assumed that failures occur because ablation is not performed at the correct site or does not create a lesion deep enough within the ventricular wall to reach the reentrant pathway.
It is extremely desireable to prognose the likelihood of a myocardial infarct patient being susceptible to ventricular tachycardia. U.S. Pat. No. 4,680,708 to M. Cain and B. Sobel suggests a method and apparatus for analyzing electrocardiogram signals to prognose ventricular tachycardia, but the early detection of myocardial infarct patients susceptible to ventricular tachycardia remains a problem.
In view of the foregoing, an object of the present invention is to provide a technique which is effective in combatting VT, does not require the administration of drugs, and does not require open-heart surgery.
A further object of the present invention is to provide a means for prognosing the likelihood of ventricular tachycardia occuring in a myocardial infarct patient not previously diagnosed as having ventricular tachycardia.